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The pizza box solar oven is a great project for kids because it demonstrates two of the three basic principles of passive solar design working in concert with each other to accomplish a goal the kids can really relate to: making something yummy!

The principles demonstrated are:

• Solar Gain - arranging for sunlight to enter a device as a source of energy. In this case, the gain is accomplished both by reflection and direct gain. This principle also includes using dark colored surfaces to absorb the solar energy that enters a device. 
• Insulation - containing heat by trapping air inside and around a device to contain heat, and reflecting thermal radiation back into a device.

The third principle of passive solar design - thermal mass, can also be experimented with the solar oven. If you are interested in this option, see the project "Build a Passive Solar Design Laboratory" for ideas. Large amounts of food will provide some thermal mass, causing the oven to heat up more slowly.

Besides explaining these principles in the process of building and using the ovens, here are several other points you might want to make:

• Cooking food takes alot of energy! By using solar energy, we can save alot on fuel.
• Cooking takes time, and the Sun will change position during that time. Therefore, somebody, such as a vigilant cook, may need to align the solar oven now and then to keep the sunlight entering. Mechanisms that track the sun and adjust the device automatically are called "heliostats" (like thermostat, but with "helio", which means "Sun", instead).
• Solar ovens have been used for a long time. In the 1830s, the British astronomer John Herschel used a solar collector box to cook food during an expedition to Africa. Nowadays, one can buy commercial solar ovens, ranging from small single dish units, to large units that can feed many people at once and that have to be hauled around on a trailer.
• Without the reflector flap, the solar oven becomes what is called a "flat plate collector". Flat plate collectors are used for many applications, such a heating water (the reason for not using a reflector is that it is not really needed for these applications- and thus alignment difficulties associated with reflectors can be avoided). One of the first known uses of solar hot boxes was by the cooks of  the Roman Emperor Tiberius who wanted to eat cucumbers all year round. The cooks satisfied his regal appetite by using a solar hot box, a kind of flat plate collector,  to grow the cucumbers all winter long! Nowadays, many people also use flat plate collectors to heat water for their pools and houses.  

The simplest pizza box solar oven design, as given below, can get up to two hundred degrees fahrenheit on a warm sunny day, enough, for example, to make "s'mores" (graham cracker sandwiches of chocolate chips and marshmellows). Several optional features will enable the oven to get even hotter, which may be desirable in cooler weather, or for more serious cooking. One should allow ample time for cooking - roughly twice as long as would take in a conventional oven, and for smore's, it works best to leave the sandwiches open while cooking so that direct sunlight falls on the marshmellows and chocolate chips). We do not recommend trying to use the oven outside in temperatures below about 60 degrees Fahrenheit. If its cool outside, try a sunny window sill.

Note: Many pizza shop owners will be more than willing to donate boxes. In return, you may want to ask a local reporter to cover the event, and ask the reporter to specifically mention the pizza shop's donation in any news article that appears.

Materials needed for a single oven (simplest design)

• 1 large size pizza box oven
• Several feet of aluminum foil
• 1 sheet black construction paper
• 2 1/2 feet of clear plastic wrap
• 4 feet of masking tape
• 2 feet of string

Note: Avoid materials that you think might become toxic when heated

Tools needed

• scissors (teachers or older students may also want to have an exacto knife on hand, to better be able to cut cardboard with).
• ruler
• marker

Instructions

1. Assemble the pizza box, and open it up. 
2. Glue aluminum foil to all inside surfaces of the sides except the top of the box, with the shiny surface facing in. This will create a "radiation trap" that will trap, by reflection, invisible (low-frequency) radiation that is radiated by the food and air inside the box.
3. On the top flap of the pizza box draw a square with a marker with edges spaced 1" from the four sides of the box.
4. Cut along three of the lines, on the sides and on the front edge of the box, leaving the fourth line along the box's hinge uncut. Then fold open the flap, making a crease on the fourth line (see the figure above).  Note: Extra supervision make be needed during this step, because students often cut along the fourth line as well by mistake.
5. Glue aluminum foil to the inside surface of the top flap, with shiny side visible! This will form reflector, to reflect sunlight into the oven. Be careful to make as few wrinkles as possible, and smooth out whatever wrinkles occur.
6. Tape the black construction paper to the bottom of the box. This will help to absorb the incoming sunlight.
7. Carefully stretch the plastic wrap over the opening of the box, sealing the edges with tape to seal the air in. 
8. Cover any air leaks around the box edges with tape, except while making sure that the box can still be opened, so you can place food inside the box and remove it later.
9. Go outside in the sunlight and place oven on a flat, level surface.
10. Place food on some foil (or a paper plate) and place inside the oven. 
11. Use string and masking tape to tie back and adjust the reflector, so that sunlight is reflected into the oven, and especially onto the pie tin.
12. Let food cook, and check reflector angle now and then to make sure sunlight is getting inside the oven.
13. Enjoy your solar treat! 

Optional Features

• Add addition flaps to reflect sunlight into the oven. This can substantially increase the gain of the oven. This will require some extra cardboard (from some old boxes for example), and some extra foil, glue, and string to adjust the flaps.
• Crumple up some sheets of newspaper and stuff them around the inside of the box, to provide extra insulation.
• Add an additional layer of saran wrap across the box opening, but attached to the inside surface of the top flap, such that an air space is created between the layers of wrap (the plastic is bound to stick together in some places: don't worry about this too much).
• Place a thermometer inside the oven as well, to measure the temperature.

The earliest pizza box solar oven design we are aware of was created in 1976 by Barbara Kerr.

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